The acetabulofemoral joint, commonly referred to simply as the hip joint, is a ball-and-socket joint that is formed through articulation of the rounded, ball-shaped head of the femur (commonly referred to in the art as the femoral head) within the cup-like hip socket, or acetabulum, that is formed in the pelvic bone. Functional limitations and discomfort in the hip joint can result from a myriad of different factors, such as arthritic conditions, degeneration of the femoral and pelvic bones and/or physical trauma. In extreme circumstances, regression of the hip joint ultimately necessitates the implantation of prosthetic components to restore useful functionality of the hip joint.
For instance, total hip replacement surgery is an increasingly common surgical procedure that involves replacement of the femoral head and the acetabulum with corresponding implantable prosthetic components. In the first step of the surgical process, the patient is disposed on a surgical table in either the supine position (i.e., lying face up with the anterior aspect of the pelvis disposed directly upward) or the lateral decubitus position (i.e., with the hip joint to be repaired facing directly upward). The patient is then firmly retained in place using a hip positioning device that is fixedly secured to the surgical table. One type of hip positioning device which is well known in the art to position the patient in the lateral decubitus position includes a pair of opposing support members that firmly bear against the anterior and posterior aspect of the pelvis. Together, the support members serve to hold the pelvis stable during the hip replacement procedure.
Upon incising the patient, the ligaments and muscle in the hip joint region are separated to permit access to the hip joint. The native femoral head is then severed from the remainder of the femur by cutting through the femoral neck using an appropriate surgical instrument, such as a saw.
With the native femoral head removed therefrom, the acetabulum is then reconstructed. Specifically, the hip socket is first reamed to remove articular cartilage and shape the pelvic bone in its desired configuration (i.e., as a smoothed, hemispherical, cup-like socket). After completion of the reaming process, a prosthetic, cup-like, acetabular component (also referred to in the art simply as an acetabular cup) is removably mounted onto the proximal end of an elonaged, rod-shaped implantation instrument that is commonly referred to in the art as an inserter. Holding the distal end of the inserter, the acetabular component is aligned within the reamed portion of the pelvis.
The acetabular component is often fittingly retained in place, or impacted, within the reamed acetabulum by forceably striking the distal end of the implantation with a mallet or other similar instrument. Various means are used to additionally secure the acetabular component within the hip socket and include, inter alia, (i) applying an adhesive between the acetabular component and the pelvic bone, (ii) driving fastening elements (e.g., screws) through the acetabular component and into the pelvic bone and (iii) roughening the exterior surface of the acetabular component to promote frictional engagement with the pelvic bone.
Once implantation of the acetabular component is completed, the proximal end of the femur is hollowed out so as to define a canal that is dimensioned to fittingly receive the elongated metal stem for the femoral component, the stem being preferably retained therein using any combination of adhesives, fastening elements and/or frictional engagement. With the stem of the femoral component retained within the femur, the prosthetic femoral head (typically constructed as a metal ball) is secured onto the free end of the stem. Once implantation of the femoral and acetabular prosthetics has been completed, the artificial femoral head is disposed within the artificial acetabulum, thereby completing reconstruction of the acetabulofemoral joint.
The success rate associated with total hip replacement surgery has been found to be largely dependent upon a number of relevant factors. In particular, it has been found that proper orientation of the implanted acetabular component within the reamed pelvic region is critical to the overall success of the surgery. Improper orientation of the acetabular component can lead to, among other things, component impingement as well as dislocation of the hip joint, which is highly undesirable. Accordingly, it is to be understood that implanting the acetabular component in the proper orientation relative to the pelvis is of paramount importance.
Traditionally, proper orientation of the acetabular component is accomplished by the surgeon by identifying certain anatomical landmarks on the pelvis and, in turn, visually estimating the proper angle of orientation relative thereto. Referring now to FIG. 1, there is shown a front perspective view of a normal human pelvis 11 that is oriented such that the right acetabulum 12 is readily identifiable. As can be seen, pelvis 11 includes three imaginary orthogonal reference planes that are defined using specified anatomical landmarks. Specifically, the left anterior superior iliac spine (ASIS) 13, the right anterior superior iliac spine 15 and the pubic symphysis 17 together define the anterior pelvic plane 19. The anterior pelvic plane 19 extends vertically when the patient is standing upright and runs generally in parallel with the coronal plane of the body (i.e., the vertical plane that separates the body into ventral and dorsal sections). The transverse plane 21 extends horizontally when the patient is standing upright and extends from the left side of pelvis 11 to the right side of pelvis 11. The sagittal plane 23 extends vertically when the patient is standing upright and extends from the front, or anterior, of pelvis 11 to the back, or posterior, of pelvis 11.
Acetabular components are typically constructed from one or more pieces that together create a generally hemispherical cup. As such, it is to be understood that an acetabular component is shaped to define an imaginary center axis that extends through its apex. It is the relationship between the center axis for the acetabular component and the orthogonal planes of pelvis 11 that is used to properly orient the acetabular cup during total hip replacement.
There are two primary angles that are used to properly orient an acetabular component within reamed acetabulum 12, namely, (i) abduction and (ii) anteversion. Abduction of the acetabular component is the angle between transverse plane 21 and the main axis for the acetabular cup as projected onto anterior pelvic plane 19. Anteversion of the acetabular component is the angle between anterior pelvic plane 19 and the main axis for the acetabular cup as projected onto transverse plane 21. Traditionally, the ideal orientation of an acetabular cup within reamed acetabulum 12 is defined as approximately 40-50 degrees of abduction and approximately 15-25 degrees of anteversion.
Visual estimation of the abduction and anteversion angles has been found to be very difficult to assess during total hip replacement since the aforementioned pelvic reference planes are imaginary in nature. In addition, it should be noted that the pelvis is encased in soft tissue and is typically covered with surgical drapes during the procedure, thereby rendering visualization of the pelvic reference planes nearly impossible to achieve. Furthermore, even if the orientation of the pelvis is accurately determined, the ability of the surgeon to visually estimate instrument angles relative thereto is highly subjective and therefore prone to error.
In view of the aforementioned shortcomings associated with using visual estimation means for determining proper orientation of the acetabular cup, mechanical alignment guides have been recently developed for use in connection with hip replacement surgeries. Mechanical alignment guides provide the surgeon with a visual reference of the orientation of the acetabular component relative to the plane of the patient and operating room table. Although well known and widely used in the art, mechanical alignment guides of the type as described above have been found to have the potential to introduce significant error. In particular, the use of mechanical alignment guides requires that the pelvis be positioned properly prior to draping the patient for surgery, which is not always achieved. In addition, mechanical alignment guides fail to account for any intraoperative changes in pelvic orientation.
Accordingly, electronic surgical guidance systems are well known in the art and are used to assist in properly orienting prosthetic components, such as acetabular components, within a patient. Electronic surgical guidance systems allow for a target, or ideal, orientation angle for the acetabular component to be input into a central controller. Using spatial orientation data derived from sensors mounted on the implantation instrument, the controller provides feedback signals to the surgeon that indicate when the acetabular component is disposed at the desired angle of orientation.
For example, in U.S. Patent Application Publication No. 2010/0249796 to J. H. Nycz, which is incorporated herein by reference, there is disclosed a surgical instrument for implanting a prosthetic member. The instrument includes an orientation sensor that detects an initial orientation of the prosthetic member and an implanting orientation of the prosthetic member. The instrument further includes a memory module and an input device that receives a user input to transfer the initial orientation detected by the orientation sensor into the memory module for storage. Furthermore, the instrument includes an orientation feedback device that selectively provides an orientation feedback signal to the user. Moreover, the instrument includes a controller that causes the orientation feedback device to provide the orientation feedback signal when the implanting orientation detected by the orientation sensor is substantially equal to the initial orientation stored in the memory module.
Although well known in the art, surgical guidance systems of the type described above have been found to suffer from a few notable shortcomings.
As a first shortcoming, surgical guidance systems of the type described above are rather complex in their construction and use. As a result, it has been found that the costs associated with such systems are largely prohibitive. In addition, the technical expertise that is required to operate such a system is rather significant and lacks user-intuitiveness and thereby necessitates that the surgeon overcome a considerable learning curve prior to first use, which is highly undesirable.
As a second shortcoming, surgical guidance systems of the type as described above typically require that a reference point, or marker, be electronically identified in the pelvis as part of lengthy preparatory step. Once the reference marker in the pelvis is electronically established, orientation of the instrument is then standardized, or calibrated, relative to the reference marker. As can be appreciated, the aforementioned multi-stepped calibration process has been found to be both time-consuming and labor-intensive in nature, which is highly undesirable.